1. Field of the Invention
The present invention relates to disk for a toroidal continuously variable transmission which is used as a transmission mainly for an automobile or for various industrial machinery, as well as a method of machining the same.
2. Description of the Related Art
In recent years, as variable transmissions for use in automobiles or the like, toroidal continuously variable transmissions are used. In addition, to cope with a greater input torque, double cavity-type toroidal-type continuously variable transmissions are used in which two sets of continuously variable transmission mechanisms are provided.
Conventionally, with this type of toroidal-type continuously variable transmission, both output disks of the two sets of continuously variable transmission mechanisms are disposed in close proximity to a periphery of an input shaft, and the rotation of the input disk is transmitted to the output disks by means of power rollers. In addition, an output gear is coupled between the both output disks, and mutually synchronized rotations of the both disks are transmitted to the output gear.
Meanwhile, a technique is known in which to make the above-described toroidal-type continuously variable transmission in order to be lightweight, an output gear for transmitting a driving force on an outer periphery of the output disk to fetch the power from the output disk (e.g., refer to Japanese Patent Unexamined Publication JP-A-8-159229). In addition, there is a type in which a pair of output disks are formed of the same material and are constructed as an integral-type output disk, and the length of the toroidal-type continuously variable transmission is made compact. It is known that a surface having concave and convex portions, which is constituted by an output gear for transmitting the driving force and sensor grooves or the like for detecting the state of rotation, is also provided on the outer periphery of such an integral-type output disk (e.g., refer to U.S. Pat. No. 4,872,371 and JP-A-11-63139). The output disk disclosed in U.S. Pat. No. 4,872,371 has traction surfaces on its both surfaces, and has an output gear formed integrally on its outer periphery for transmitting the driving force. Meanwhile, in the case of the output disk disclosed in JP-A-11-63139, an integral-type output disk 103 having traction surfaces 101 and 102 on its both surfaces and an output gear 104 are machined separately, and are subsequently integrated by welding or the like, as shown in FIG. 11.
In addition, since the disk having the traction surfaces has a function of transmitting the driving force while rotating at high speed, if the disk runs out when rotating, drawbacks can occur in that the state of speed change becomes unstable and the disk fails to synchronize. For this reason, it is known that the amount of runout of the traction surface is set to 0.02 mm or less (e.g., refer to JP-A-11-148543). Accordingly, also with the integral-type output disk having traction surfaces on its both surfaces, there is a need to increase the accuracy in the concentricity of the traction surfaces on the both surfaces and the accuracy in the concentricity of each traction surface with respect to portions where the disk is rotatively supported (mainly the inner peripheral surface).
Furthermore, various methods have been proposed as methods of machining a disk for a toroidal-type continuously variable transmission (e.g., refer to JP-A-2002-28818, U.S. Pat. No. 6,663,471, JP-A-2000-271844, JP-A-2000-24899 and JP-A-2002-192450). In the machining method disclosed in JP-A-2002-28818, the traction surface is machined by using as a machining reference spline tooth surfaces at the inner peripheral surface of the disk. In addition, in the machining method disclosed in U.S. Pat. No. 6,663,471, as shown in FIG. 12, there are provided a chuck mechanism 205 for holding a CVT disk 204 which has an inner surface involute spline portion 201, an outer peripheral surface 202, and a traction surface 203 having a predetermined machining allowance, as well as a grinding mechanism 207 equipped with a grinding wheel 206 for grinding the CVT disk 204. Grinding is performed by tilting either one of the CVT disk 204 and the grinding wheel 206 with respect to the other. At this juncture, chucking a part of the inner surface involute spline portion 201 by the chuck mechanism 205, and the traction surface 203 is ground, and at the same time an inner surface portion 208 is concurrently ground in combination by an inner surface grinding wheel 210 fitted to an inner surface grinding spindle 209. Therefore, high accuracy in concentricity is ensured between the traction surface 203 and the inner surface portion 208.
In addition, with the integral-type output disk, to maintain the accuracy in the concentricity of both traction surfaces and the accuracy in the concentricity of each traction surface and the inner peripheral surface to high levels, a machining method such as the one shown in FIGS. 13 and 14 is known.
First, as shown in FIG. 13, one flat surface portion 302a of an integral-type output disk 301 is abutted against a reference washer 303, and an electromagnetic force is applied to the reference washer 303 to fix the disk 301. In addition, an outer peripheral surface 304 of the disk 301 is abutted against a pair of shoes 306 fixed to a supporting base 305, so as to position the disk 301 in the radial direction. Then, the outer peripheral surface 304 of the disk 301 rotates integrally with the reference washer 303 while sliding on the shoes 306, and its inner peripheral surface 308 is ground by an inner periphery grinding wheel 307 which is rotating.
Next, as shown in FIG. 14, in the state in which the disk 301 is being supported in the same way as in FIG. 13, one traction surface 310a is machined by a traction surface grinding wheel 309. In addition, after the one traction surface 310a has been machined, the disk 301 is inverted, and the other flat surface portion 302b is abutted against the reference washer 303 to fix the disk 301, and the other traction surface 310b is machined by the traction surface grinding wheel 309.
Accordingly, the grinding work of the both traction surfaces 310a and 310b and the inner peripheral surface 308 is performed while the outer peripheral surface 304 is abutted against the shoes 306 to effect radial positioning and while the disk 301 is being rotatively driven by the reference washer 303 abutting against the flat surface portions 302a and 302b. Here, since the disk 301 rotates while sliding without being moved away from the shoes 306, the center of rotation of the disk 301 is the outer peripheral surface. As a result, all the machined surfaces are theoretically machined concentrically with the outer peripheral surface, and the both traction surfaces 310a and 310b and the inner peripheral surface 308 can be theoretically machined concentrically with each other.
In a case where the integral-type output disk is machined by using the machining method disclosed in U.S. Pat. No. 6,663,471, the accuracy in the concentricity between one traction surface and the inner peripheral surface can be made high. However, to machine the other traction surface, it is necessary to remove the disk from the chuck mechanism once. For this reason, due to an error at the time of chucking or the biting of a foreign material or the like, there is a possibility that the chucking accuracy becomes unstable, resulting in the deterioration of the accuracy in concentricity with the one traction surface which has already been machined.
In addition, with the method of machining an integral-type disk shown in FIGS. 13 and 14, it is prerequisite that all or a portion of the outer peripheral surface of the disk be a smooth cylindrical surface. If an surface having concave and convex portions 311 constituted by a gear or sensor grooves or the like is present on the outer peripheral surface 304, as shown in FIG. 15, vibrations can occur on the supporting base 305, possibly rendering the machining impossible.
Furthermore, in a case where the integral-type disk having an surface having concave and convex portions on its outer peripheral surface is machined, after the outer peripheral surface is formed into a smooth cylindrical surface and machining is effected with high accuracy in concentricity by the machining method shown in FIGS. 13 and 14, it is conceivable to integrate a separate gear or the like by welding and press fitting, as disclosed in JP-A-11-63139. However, since the number of parts increases, the number of steps increases due to the step for integration, so that there is a problem in that the machining cost increases.